Edge coupling with edge couplers is a standard technique for coupling between single-mode fibers and photonic integrated circuit (PIC) devices such as optical switches, modulators, high-speed detectors, and interposers. Edge coupling provides a broadband response, offers low insertion loss (IL), and couples both transverse electric (TE) modes and transverse magnetic (TM) modes. Edge couplers include nano-tapered edge couplers, or spot size converters, and evanescent edge couplers. Evanescent edge couplers couple to optical fibers using a nano-taper that is coupled to a second, larger waveguide. The waveguide is formed of a polymer or an inorganic material such as silicon oxynitride (SiON) or an oxide.
Coupling efficiency is high when the mode field diameter (MFD) of a fiber and a waveguide are matched and when incoming light and outgoing light are aligned. Coupling efficiency is sensitive to misalignment, for example, between a fiber and a waveguide. As an example, a 0.5 micrometer (μm) lateral offset of an inverted-taper spot size converter (SSC) reduces a coupled light power output by half.
Optical coupling of multiple channels in an active PIC is challenging and expensive. Optical coupling of multiple channels is prone to signal drifting during the lifetime of operation. It is desirable to monitor edge coupling alignment between single-mode fibers and PICs during the lifetime of operation.